What is the reality of intention (niyyah) in acts of worship, and is a mere heartfelt resolve sufficient for it without uttering it aloud? Does this necessitate specifying the time of prayer?

The intention for prayer is made at its beginning, and it must specify the prayer (e.g., Dhuhr or Asr) in the heart, not verbally. A person does not perform any act without an intention; thus, in acts of worship, intention distinguishes them from habits. For example, a ritual bath (ghusl (definition)) for major impurity requires intention to differentiate it from a bath taken for cooling. Similarly, intention distinguishes between similar acts of worship, such as differentiating a obligatory prayer (fard) from a supererogatory one (nafl), or Dhuhr from Asr.